Stereopsis is the impression of depth that is perceived when a scene is viewed by someone with two eyes and normal binocular vision. Since the eyes of humans, and most animals, are located at different lateral positions on the head, binocular vision results in projection of slightly different images to the retinas of the eyes. The differences are mainly in the relative horizontal position of objects in the two images. These positional differences are referred to as horizontal disparities or, more generally, binocular disparities. Disparities are processed in the visual cortex of the brain to yield depth perception.
Stereopsis is not shared equally by all people. It has been found that people possess different degrees of stereo acuity, that is, different degrees to which objects may be perceived to lie in different planes when viewed binocularly. One study reports that 97% of test subjects were able to perceive depth differences at binocular disparities of 2.3 minutes of arc or smaller, and at least 80% of test subjects could perceive depth differences at a binocular disparity of 30 seconds of arc. See Coutant, B. E., et al., Ophthal. Physiol. Opt., 13(1):3-7 (1993). The inability to perceive depth of vision binocularly may be caused by conditions such as strabismus or for other less apparent reasons. Occupations requiring the precise judgment of distance sometimes include a requirement to demonstrate some level of stereopsis; in particular, this is the case for airplane pilots.
Monocular depth cues, which by definition are depth cues which only require use of one eye, include retinal image size, linear perspective, accommodation, and motion parallax.
Retinal image size permits depth judgments based on prior knowledge and familiarity with similar objects. If two objects are known to be of equal size and one appears larger than the other, than the smaller object is perceived to be further away. Similarly, if the size of an object is known, it is possible to estimate its distance by the size of its appearance.
When objects of known distance subtend a smaller and smaller angle, it is interpreted as being further away. This monocular cue, referred to as linear perspective, is commonly exemplified by the apparent convergence of train tracks as they recede into the distance.
Accomodation is an oculomotor cue for depth perception. When we try to focus on far away objects, the ciliary muscles stretch the eye lens, making it thinner, and hence changing the focal length. The kinesthetic sensations of the contracting and relaxing ciliary muscles (intraocular muscles) is sent to the visual cortex where it is used for interpreting distance/depth.
Motion parallax is created when an observer translates while viewing a rigid environment. While the observer's fixation is automatically maintained on a specific point, objects nearer or farther than the fixation point move relative to each other on the observer's retina. The visual system uses this relative movement of objects on the retina, motion parallax, as a cue to the relative depth of these objects in the environment.
Historically, laboratory stereopsis testing has been performed by measuring “real” depth perception utilizing special instrumentation. One of the oldest and best known of these “real” tests is the Howard-Dolman test. In the Howard-Dolman test the subject views two 1-cm diameter vertical rods at 6 meters through a horizontal aperture placed near the eyes that occludes the ends of the rods. The subject pulls a string attached to one rod until the two rods appear equidistant. Several monocular cues are present in this test, including the relative widths of the images of the rods and motion parallax due to head movements. Due to its large size, the Howard-Dolman device is impractical for use in clinical settings.
Another “real” test utilizes the Verhoeff Stereopter, described in Verhoeff, F H, Archives of Opthamology, 28:1000-1014 (1942). As described in the paper, the device comprises a rectangular black screen (target screen) at least 9 by 17.5 cm in size with its long axis vertical. In this is centered a rectangular window (target window) 1 by 5.4 cm in size, with its long axis horizontal. Immediately behind the window, held so that it can slide only vertically, is a smaller screen (sliding screen) 11 cm high, 6.9 cm wide and exactly 2.5 mm thick. The sliding screen contains four rectangular windows, each 16 by 50 mm in size, centered on the vertical midline with their long axes horizontal and separated from each other by distances of 5 mm. Crossing each window vertically are three thin black strips: one 3 mm in width centered exactly on the midline; one 2.5 mm in width centered 10.75 mm from the midline on one side of the 3 mm strip; and one 2 mm in width centered 10.50 mm from the midline on the other side. Of the strips, some are affixed to the back and the others to the front of the sliding screen, thereby providing a depth of 2.5 mm between the strips at the front and those at the back. By moving the sliding screen, any of the four sets of strips can be exposed in the target window, and by turning the device upside down, the positions of the lateral strips can be reversed. The sets are numbered from bottom upward. In set 1, the middle strip is at the front, the 2.5 mm strip at the back on the left and 2 mm strip at the front on the right. In set 2, the middle strip and 2.5 mm strips are at the back, and the 2.5 mm strip at the front on the left. In set 3, the middle and 2.0 mm strips are at the back, and the 2.5 mm strip is at the front on the left. In set 4, the middle strip is at the back, and the 2.5 and 2.0 mm strips (on the left and right) at the front.
Behind the target window of the Verhoeff Stereopter and about 3 mm behind the sliding screen is a stationary translucent diffusing screen 5.8 by 2 cm, which is indirectly attached to the target screen. During testing, the diffusing screen is evenly illuminated from behind, using for example, ordinary daylight from a window as a light source, or a 2.2 volt flashlight bulb and a curved reflector. Finally, Verhoeff further describes the stereopter as having a protective cover over the back of the device which is suitably marked so that each of the sets of strips can be identified from behind and, by the aid of a knob, placed in position by the examiner.
Verhoeff describes use of the stereopter for testing stereopsis as follows. During testing the device is kept centered as a frontal plane on the subject's binocular visual midline, and held steady to avoid monocular parallax. Verhoeff instructs that during testing, the target window should not be exposed while the device is being placed in position or the sets are changed. In particular, Verhoeff teaches that the examiner should grasp the device over the target window with the left hand, place the desired set into position with the right hand and then grasp the device below with the right hand and expose the target window by moving the left hand up or down.
Although the Verhoeff stercopter was commercialized, clinical practitioners considered testing using the device complicated, time consuming, requiring precise judgment and cooperation on the part of the test subject, and the device itself as cumbersome. Due to these constraints, the Verhoeff stereopter is not considered practical for use in a clinical setting. See, for example, HELVESTON, EUGENE M. “STEREOPSIS AND STRABISMUS.” In Strabismus and Amblyopia: Experimental Basis for Advances in Clinical Management (Wenner-Gren International Symposium Series, Vol 49), vol. 49, p. 359. Springer, 1988. Consequently, the Verhoeff Stereopter has not been commercially produced for over fifty years.
The only “real” test currently used in clinical practice is the Frisby stereo plate test. The Frisby test uses transparent plates of varying thicknesses which are presented to the test subject one at a time against a clear background. Each plate is divided into four quadrants. The target of the test is a circular cluster of randomly arranged arrowheads of differing size printed on one side of each plate in one of the four quadrants. On the other side of the plate similar pattern elements are printed around the target and in the other three quadrants. For stereopsis screening, a test subject is asked to identify the target quadrant upon repeated trials wherein the position of the target quadrant is randomly selected. According to the manufacturer, an observer lacking normal binocular stereovision fails to detect the target as it can be distinguished only on the basis of binocular disparity cues to depth, as long as the plate is held stationary, viewed square-on, and placed about 5-10 cm in front of a clear background.
The Frisby stereo plate test suffers from a number of deficiencies. First, the plate must be held in a particular spatial relationship to a separate plain background provided on the box which holds the plates. This requirement necessitates that the box and plate be administered while resting on a table or on the lap of the examiner. Second, consecutive presentations of the same plate require the examiner to randomly turn the plate around unobtrusively, for example, while holding the plate behind their back. This adds considerably to the amount of time required to administer the test. Finally, no means are provided to ensure optimal lighting conditions for viewing the tests, and care must be taken to avoid reflections or shadows caused by light sources behind or over the patient. This limits the settings and conditions under which the test may be administered.
While “real” tests of depth perception rely on binocular disparities created when viewing a three dimensional scene with two eyes, binocular disparities can also be simulated by artificially presenting two different two dimensional images (referred to collectively as a stereogram) separately to each eye using a method called stereoscopy. Vision care practitioners have for the past century used devices for viewing stereograms to measure stereo acuity in patients. Stereograms produced for this purpose are sold commercially by several manufacturers. The most commonly used stereograms fall into two groups, namely those featuring contours which provide monocular cues as to the form or symbol in the stereograms, and those without such cues, which latter group is known as random dot stereograms. When viewing a stereogram of this nature through dissociating polarized lenses the patient with normal stereopsis perceives images of objects in the stereogram to be displaced either forwardly or rearwardly of the plane of regard or fixation being looked at. This illusion is achieved by creating on the stereogram a form having stereo disparity. The larger the degree of stereo disparity of the symbol being observed, the further it appears to be displaced relative to the plane of regard and hence the easier will it be for the patient to discriminate between the form and its reference ground. The ability to see the apparent displacement can hence be used as a measure of the patient's stereo acuity.
Some practitioners favor use of stereogram-based stereoscopic tests since by design such tests eliminate monocular (non-stereoscopic) depth cues typically present in “real tests,” such as accommodation, motion parallax resulting from head movements, texture perspective, and/or the relative widths of stimuli based on size of the retinal image. Notwithstanding this perceived advantage, care must be taken with interpreting the results of stereogram-based tests since some people with otherwise normal stereoscopic vision have difficulty fusing random-dot stereograms, especially if they they cannot correctly focus on the stimulus.
Depth perception is an important skill needed in many facets of life, such as driving, flying, and in many occupations and leisure activities. In performing daily activities, depth perception is the result of stereoscopic vision, utilizing both eyes, and/or monocular vision skills.